Small axial flow fan motors have been installed in electronic devices such as personal computers, servers, and copiers for cooling casings or for cooling individual electronic components.
An example of an axial flow fan motor of this type was described in Japanese Unexamined Utility Model Application No. H7-36573 (hereinafter “Patent Reference 1”). In this motor, as shown in FIG. 6, a shaft 106 of an impeller 109 made from a synthetic resin and having a plurality of fans 111 is rotatably supported by bearings 104 and 105 in a bearing box 103 of a casing 101 forming an inner tubular venturi portion 102. A stator 120, composed of a core 117, a coil 118, and an insulator 119, and a PC board (printed circuit board) 115 having drive circuits for the motor mounted thereon are installed at an outer side of the bearing box, while a rotor composed of a yoke 113 and a magnet 114 is installed at the impeller side in an opposing relationship with the stator.
If an electric current is passed to the drive circuits, the stator 120 generates a magnetic field, a rotary force acts upon the rotor, the impeller 109 rotates, and a unidirectional airflow is generated inside the venturi portion 102 of the casing 101.
Furthermore, as described in Patent Reference 1, the impeller 109 and the shaft 106 thereof are formed integrally from synthetic resin in order to reduce cost by reducing the number of parts and assembly operations.
However, in the motor described in Patent Reference 1, the strength of the shaft 106, which is molded from a synthetic resin integrally with the impeller 109, is weak compared with that of metal shafts. For this reason, Japanese Utility Model Registration No. 3028698 (hereinafter “Patent Reference 2”) described an axial flow fan motor, as shown in FIG. 7, in which, the reinforcement was made by installing a metal rod 221 inside the shaft portion 206A during molding of the shaft-integrated impeller 209 from a synthetic resin to increase the strength of the shaft 106.
However, in the axial flow fan motor described in Patent Reference 2, in order to install the metal rod 221 inside the shaft portion 206A, it is necessary to set the metal rod 221 in the cavity of the molding die to form the shaft-integrated impeller 209. This molding operation is more difficult than the operation of molding the shaft-integrated impeller 109 described in Patent Reference 1.
Furthermore, it was preferred in both in the axial flow fan motors described in Patent Reference 1 and in Patent Reference 2, that tight contact be maintained and the components be fixed by press fitting into the inner rings of rolling bearings 104 (204), 105 (205) by making use of the elastic properties of the shaft formed from a synthetic resin. (In such a case, the adhesive is not required and the motor can be assembled and disassembled). However, taking into account the difficulty of press fitting and possible damage of rolling bearings at the time of press fitting, the aforesaid press fitting was avoided and the shaft and inner rings of rolling bearings 104 (204), 105 (205) were fixed by employing clearance fitting and using a locking ring 107 (207).